1. Field of the Invention
The present general inventive concept relates to an image transfer device of an image forming apparatus. More particularly, the present general inventive concept relates to an image transfer device which accurately measures a resistance of a printing medium or an endless track belt, and an image forming apparatus having the same.
2. Description of the Related Art
A conventional laser beam printer includes an image forming means such as a paper cassette, a developing device and a fuser, and carries out a printing operation by attracting toner onto a paper sheet.
The operation of a laser beam printer can be mainly categorized into charging, writing, developing, transferring, and fusing. When charging, a charger is energized to a high voltage of approximately 7000V and forms a minus (−) charge on the surface of a photoconductor drum by a Corona discharge. When writing, a laser beam scans the surface of the photoconductor drum bearing the (−) charge so that a desired electrostatic latent image is formed according to the (−) charge pattern. When developing, a developing roller and photoconductor drum are rotated in intimate contact with each other so that toner particles of the (−) charge are attracted onto the electrostatic latent parts. When transferring, a transfer device is energized to a predetermined voltage and the other side of the paper sheet has a (+) charge so that toner particles of the (−) charge are attracted onto a paper sheet when the paper sheet is passed between the photoconductor drum and the transfer device. When fusing, the toner image is fixed into the paper sheet by appropriate heat and pressure. Accordingly, the paper sheet bearing the final image is released.
Meanwhile, each type of printing medium has a different resistance depending on the thickness, wetness, or quality, and the printing quality heavily depends on the resistance. An endless track belt also has a resistance which varies depending on manufacturing processes or use.
To prevent image quality degradation, a conventional image forming apparatus includes a roller which is connected with a resistance measuring circuit, so that a resistance of a paper sheet or an endless track belt can be measured and an associated transfer voltage can be transmitted to the transfer roller before an image transfer.
However, because there are different types of materials included in an inner side of a resistance measuring roller, resistance of the resistance measuring roller varies in the circumferential direction. A conventional image forming apparatus fails to reflect a potential varying resistance of the paper sheet or the endless track belt, because the interval between the resistance measuring roller and the transfer roller is shorter than the circumference of the resistance measuring roller, which means that the varying resistance in the circumferential direction of the resistance measuring roller is not completely considered.